Simultaneous assessment of membrane bilayer structure and drug insertion by 19F solid-state NMR

Fluorine-19 is an ideal nucleus for studying biological systems using NMR due to its rarity in biological environments and its favorable magnetic properties. In this work, we used a mixture of monofluorinated palmitic acids (PAs) as tracers to investigate the molecular interaction of the fluorinated drug rosuvastatin in model lipid membranes. More specifically, PAs labeled at the fourth and eighth carbon positions of their acyl chains were coincorporated in phospholipid bilayers to probe different depths of the hydrophobic core. First, the 19 F chemical shift anisotropy (CSA), indicative of membrane fluidity, was simultaneously determined for fatty acids (FAs) and the fluorinated drug using either slow magic-angle spinning (MAS) 1D 19F solid-state NMR (SS-NMR) or MAS 2D 19 F-19F SS-NMR with CSA recoupling. Membrane heterogeneity and selective partitioning of rosuvastatin into fluid regions could thus be evidenced. We then examined the possibility of mapping intermolecular distances in bilayers, in both the fluid and gel phases, using 19 F-19F and 1 H-19F correlation experiments by SS-NMR using MAS. Spatial correlations were evidenced between the two PAs in the gel phase, while contacts between the statin and the lipids were detected in the fluid phase. This work paves the way to mapping membrane-active molecules in intact membranes, and stresses the need for new labeling strategies for this purpose. SIGNIFICANCE Lipid membranes ensure cell integrity and protection from their environment. They can be targeted or crossed by bioactive molecules, such as drugs, on their way to the target site. In this work, we exploit the high sensitivity of fluorine-19 and its low abundance in biological systems to study the membrane state and interactions with the fluorinated drug rosuvastatin at the nanoscopic level using 19 F solid-state NMR. We demonstrate that incorporating monofluorinated fatty acids at different positions on their acyl chains into lipid bilayers allows determining membrane fluidity. Moreover, we show that membrane-active molecules can be mapped within the bilayer through intermolecular 19 F-19F and 19 F-1H contacts. This approach could be applied to study intact cells.